FLORA OF SCOTTISH LAKES 
163 
depth in Loch Ness and others is from 7 to 8 feet ; in many peaty 
lochs, however, this depth is considerably less. This multiplier, 
however, does not hold where the multiplicand is considerably greater. 
Thus at Loch Fiart, on the island of Lismore, the bottom may be 
seen at a depth of 25 feet under the above conditions, but plants do 
not thrive there at a greater depth than 45 feet. Possibly this is 
because the less refrangible rays of the spectrum, which are most 
necessary for photosynthesis, become insufficient at greater depths, 
although the rays of shorter wave-length may penetrate to greater 
depths in sufficient quantity to fulfil the requirements of the metabolic 
activities that are dependent upon them. It must be borne in mind 
that the yellow -brown colour of the water of peaty lakes probably 
neutralises the photo-chemical action of the violet rays at no great 
depth. It is known that Rhodophyceae thrive in the sea to at least 
a depth of 250 feet, but in all probability their reddish colour 
accentuates the photo-chemical action of the very feeble yellow-green 
rays ^ that penetrate such a depth of water. In similar manner a 
photographic plate becomes more sensitive to certain rays when its 
film is stained with suitable colours. Thus a film stained with 
erythrosin becomes sensitive to green and yellow. Exact information 
on these points in various waters of Scotland is much needed. 
In the littoral region of the sea are found well-marked zones of 
vegetation, in which the plants of one trespass but little upon the 
domain of the others. This is to a great extent dependent upon the 
rise and fall of the tide. In fresh water, however, such well-differ- 
entiated zones cannot be distinguished. Still it appears that certain 
plants usually grow in certain relative positions, where some species 
form distinct colonies ; but many others frequently, and in fact 
generally, encroach very much upon one another (vide Table, p. 193). 
It is, perhaps, convenient to imagine a set of zones for both the semi- 
aquatic and the submerged flora of a loch, but the plants are in no 
way enslaved to any set rule, excepting that many are restrained 
by certain oecological and physical conditions as mentioned hereafter 
(p. 164). As an example of a species that readily adapts itself to 
varying conditions, mention may be made of Fontinalis antipyretica. 
This plant grows in water of all depths down to 40 feet, as at Lismore ; 
it also grows equally well in the rocky bed of a burn that has water 
in it only during floods, as near Inchnacardoch Bay, Loch Ness ; again, 
it is frequently found in swiftly running streams, as well as near 
waterfalls. 
As has been already indicated, a most important factor in the dis- 
1 The rays for maximum photosynthesis in the red seaweeds are the yellow- 
green ; these penetrate water sufficiently for photosynthesis to about five times the 
depth that red rays do. 
